I am looking for a dataset on the real speed of cars in German autobahns.
This question was inspired by https://travel.stackexchange.com/q/139723/1810.
You could look into the HighD dataset:
The highD dataset is a new dataset of naturalistic vehicle trajectories recorded on German highways. Using a drone, typical limitations of established traffic data collection methods such as occlusions are overcome by the aerial perspective. Traffic was recorded at six different locations and includes more than 110 500 vehicles. Each vehicle's trajectory, including vehicle type, size and manoeuvres, is automatically extracted.
The data is collected at different locations near Cologne in Germany, hopefully one of them is at a location where there is no speed limit:
Additionally, you could look for data from induction loop detectors (Induktionsschleifendetektoren in German). These detectors record passing vehicles, from which the (average) speed and flow can be derived.